1. Field of the Invention
The present invention relates to an ion-permeable diaphragm for use in alkaline water electrolysis apparatus, and more particularly to an ion-permeable diaphragm for use in alkaline water electrolysis apparatus having a structure in which an ion-permeable diaphragm is sandwiched between electrodes.
2. Description of the Related Art
In the context of the current energy situation, hydrogen is gathering widespread attention, for a number of reasons, as a new energy source for replacing petroleum. Examples of industrial hydrogen manufacturing methods include, for instance, gasification of coke or petroleum, and water electrolysis.
The former method involves complex operations and requires extremely large facilities, and is thus problematic in terms of initial cost.
On the other hand, the latter method uses water, which is readily available, as a raw material. In water electrolysis, a plurality of electrode pairs are provided in an electrolytic bath that is partitioned by ion-permeable diaphragms, through which an alkaline electrolyte such as KOH or the like can pass, disposed between the electrodes that form the electrode pairs. Hydrogen is generated on the cathode side and oxygen on the anode side of the ion-permeable diaphragms. The ion-permeable diaphragm and the solution to be electrolyzed, however, are interposed between the electrodes. This results in greater electric resistance and worse electrolysis efficiency, which is problematic. Nevertheless, such water electrolysis methods are workable in that they can generate hydrogen even with relatively small-scale apparatus, and are thus promising in terms of enhancing electrolysis efficiency.
The membranes used in electrochemical electrolytic baths, typified by such alkaline water electrolysis apparatus, must have the following characteristics.
(1) The membrane must let through only ions, while gas must not diffuse or pass through the membrane.
(2) The membrane should be physically and chemically durably in the electrolyte.
(3) The membrane should have low electric resistance.
Asbestos yarn is widely used in practice in electrolysis diaphragms having the above characteristics. Depending on the circumstances, however, the temperature of the electrolyte may rise to 100° C. or higher. Asbestos yarn corrodes at temperatures of 100° C. or above, and cannot then be used. Moreover, the health hazards posed by asbestos have been extensively reported, and thus the use of asbestos is fraught with significant problems.
For instance, an ion-permeable diaphragm using a polymeric porous membrane or an ion-exchange membrane, and a metal oxide membrane of NiO or the like (Japanese Examined Patent Application Laid-open No. S62-50557), as well as an ion-permeable diaphragm comprising, as a membrane material, for instance a composite material of an inorganic substance and an organic polymer (Japanese Patent No. 2604734) have been proposed as ion-permeable diaphragms satisfying the above characteristics (1) and (2), and having yet lower electric resistance (3).
Among the ion-permeable diaphragms comprising the above membrane materials, polymeric porous membranes are advantageous in being flexible and having strong resistance against mechanical damage. The polymer materials used, however, are hydrophobic, and hence the solvated ions move with difficulty in the electrolyte, even in the case of a porous membrane, so that electric resistance becomes substantial. The characteristics of the electrolytic bath are thus severely impaired, which is problematic. In alkaline water electrolysis apparatus for generating a gas, polymeric porous membranes and ion-exchange membranes are problematic in that gas bubbles adhere to the surface of the membrane, thereby greatly increasing electric resistance. In particular, the membrane can deteriorate on account of the formation of high-temperature portions, called hot spots, when bubbles aggregate and give rise to local large increases in electric resistance.
Metal oxide membranes of NiO or the like, such as the one disclosed in Japanese Examined Patent Application Laid-open No. S62-50557, are manufactured by sintering. Dense sintered diaphragms, which do not allow a gas to pass or diffuse, are problematic in terms of membrane size limitations, which make sintered diaphragms unsuitable for large electrolytic baths.
The ion-permeable diaphragm disclosed in Japanese Patent No. 2604734, in which the diaphragm comprises a composite material of an inorganic substance and an organic polymer, is an ion-permeable diaphragm where micropores are formed through membrane formation using zirconium oxide or polyantimonic acid as an inorganic wettable material, and using, for instance, a fluorocarbon copolymer or a polysulfone as a binder. The ion-permeable diaphragm using such a composite material exhibits excellent smoothness and extremely good ion conductivity, and is thus appropriate as a diaphragm for alkaline water electrolysis devices.
Although the inorganic wettable material used in the ion-permeable diaphragm disclosed in Japanese Patent No. 2604734 is hydrophilic, a yet more hydrophilic inorganic material is desirable in terms of reducing the electric resistance of the ion-permeable diaphragm, which offers thus room for improvement.